1. Field of the Invention
The present invention relates generally to a system for detecting combustion state in automotive internal combustion engines. More specifically, the invention relates to a combustion state detecting system which monitors cylinder pressures to diagnose or detect combustion state, such as misfiring, in internal combustion engines.
2. Description of the Background Art
Various systems for detecting cylinder pressure (pressure in a combustion chamber) to diagnose or detect a combustion state, such as occurrence of misfire, in an internal combustion engine have been proposed. Generally, the proposed systems can determine an actual combustion state by deriving a maximum cylinder pressure or a mean effective pressure Pi in a particular engine running condition. One such combustion state detecting systems is described in "Internal Combustion Engine" (published on September 1962 by Sankaido Co., Ltd.; VOL. 1, page 19).
In this system, cylinder pressures for the respective cylinders are detected for intervals of two engine revolutions, e.g. 720.degree. of a crankshaft in the case of a four-cycle engine, by means of cylinder pressure sensors installed in respective engine cylinders, to derive the mean effective pressure Pi on the basis of the detected cylinder pressures.
Many engines installed on vehicles are multiple cylinder type engines which have four or more cylinders. If the aforementioned combustion state detecting system is applied to such a multiple cylinder type engine, there are disadvantages. That is, when cylinder pressures for the respective cylinders are detected at intervals of two engine revolutions to derive a mean effective pressure Pi for the respective cylinders, if a microcomputer is used, a high-speed A/D converter for converting the detected cylinder pressures from analog to digital and a high-speed CPU for deriving the mean effective pressures Pi must be used for the system. This causes the system to become expensive.
In order to eliminate the aforementioned disadvantage, Japanese Patent First (unexamined) Publication (Tokkai Sho.) No. 62-265548 discloses an improved combustion state detecting system. This system is basically designed to derive a pseudo-Pi (mean effective pressure) from cylinder pressures P which are detected at the interval of a predetermined crank angle, e.g. 180.degree., and fine variations .DELTA.V of cylinder volume at every cycle of the predetermined crank angle, in accordance with the following equation. ##EQU1##
Japanese Patent First (unexamined) Publication (Tokkai Sho.) No. 62-238433 discloses another combustion state detecting system, which is designed to derive a pseudo-Pi for an interval in which cylinder pressure is greater than a predetermined value, in accordance with the aforementioned equation. In addition, Japanese Patent First (unexamined) Publication (Tokkai Sho.) No. 62-238434 discloses another combustion state detecting system, which is designed to derive a pseudo-Pi for an interval in which cylinder pressure is greater than a predetermined value after a spark ignition timing, in accordance with the aforementioned equation.
However, since these systems derive a pseudo-Pi according to an interval of a predetermined crank angle, there is a disadvantage as follows.
That is, when a combustion state becomes worse so that the combustion time becomes longer as shown by a chain line of FIG. 1, a relationship between the pseudo-Pi and the mean effective pressure Pi for the interval of 720.degree. of crank angle greatly deviates from the original relationship therebetween as shown in FIG. 2. This causes the detection accuracy of combustion state to become worse. As a result, there is a disadvantage in that the control accuracy for spark ignition timing, an air/fuel ratio, an exhaust gas circulating amount and so forth also becomes worse.
Furthermore, Japanese Patent First (unexamined) Publication (Tokkai Sho.) No. 62-203036 discloses another combustion state detecting system, which is designed to detect actual cylinder pressures in a predetermined region near a compression stroke top-dead-center (TDC) position of the crankshaft, and to retrieve cylinder pressures in the other regions from a map in accordance with engine running conditions, such as the intake air flow rate, the opening angle of a throttle valve, the engine revolution speed. On the basis of these cylinder pressures, the mean effective pressure is derived for an interval of 720.degree. of the crank angle.
However, since the combustion state always varies, it is difficult for the aforementioned system which retrieves cylinder pressure from a preset map, to predict or forecast actual cylinder pressures. Therefore, in the aforementioned system, there is a disadvantage in that the detection accuracy of Pi or combustion state may become worsened.
In addition, Japanese Patent First (unexamined) Publication (Tokkai Sho.) No. 53-129717 discloses a misfire detecting system which integrates a cylinder pressure detected by a cylinder pressure sensor over two predetermined intervals before and after a compression stroke top-dead-center (TDC) position of the crankshaft, to determine an occurrence of misfire when a difference between these integrated values becomes less than a predetermined value.
Japanese Patent First (unexamined) Publication (Tokkai Sho.) No. 57-113344 discloses another misfire detecting system which integrates the detected cylinder pressure over two predetermined intervals before and after a compression stroke top-dead-center position, to determine an occurrence of misfire when a ratio of one integrated value to the other integrated value becomes less than a predetermined value.
As mentioned above, such conventional misfire detecting systems determine an occurrence of misfire by comparing the integrated values of the cylinder pressure over predetermined intervals before and after compression top-dead-center. Therefore, there is a disadvantage in that the systems may mistakenly an occurrence of misfire in the case of an abnormal combustion in which the peak of cylinder pressure is positioned at the compression stroke top-dead-center position as shown by a chain line of FIG. 3 due to over-advancing of the spark ignition timing.
In addition, in a case where these systems determine an occurrence of misfire by means of a microcomputer, there is a disadvantage in that, when delay of combustion becomes great due to delay of spark ignition timing and so forth, it is difficult to discriminate the delay of combustion from an occurrence of misfire, which causes misfire detection accuracy to become worse, since the sampling period of time is limited to 720.degree. divided by the number of cylinders. In the case of a four-cycle engine the sampling time becomes insufficient. That is, in such a limited sampling period, the difference between variations of cylinder pressure when misfire occurs and when combustion greatly delayed, is small as shown in FIG. 4. Therefore, the difference between the integrated values of these cylinder pressures is small (see a portion shown by slash marks in FIG. 4), so that it is difficult to discriminate an occurrence of misfire from delay of combustion.
Furthermore, Japanese Patent First (unexamined) Publication (Tokkai Sho.) No. 57-114837 discloses another misfire detecting system which predicts an integrated value of cylinder pressure when misfire occurs, by multiplying cylinder pressure before ignition or its integrated value by a predetermined coefficient.
However, in this system, there is disadvantages in that it is difficult to set the coefficient to the optimum, and that the sampling period of time is limited, which causes degradation of misfire detection accuracy.